A bag of biochar, 1 cubic foot, produced by Josiah in Kapoho, available from him and retail at nursery stores in Hilo and Kea'au
First trial here (in Ahualoa). Kiln and retort vessels of inexpensive materials (just CMUs and a common steel barrel)
Detail showing how the roof of 4x8x16 CMUs are held up by rebar
Barrel full of 130 lbs of split, dried Euc. robusta firewood
Barrel turned end-over-end to get it into the kiln
Added firewood around the barrel, closed up with blocks leaving air intake, and lit
Next day, removed the door blocks to look inside
Significant charring, and the residue (wood vinegar aka pyroligneous acid) show that significant offgassing of volatiles did happen
On this first trial run, the kiln didn't heat enough for 100% charring. Sides and ends are charred, not all the way through.
Upon detailed inspection, the April test burn actually gave good results. Four white buckets are completely charred material, two orange buckets incomplete, one mixed and one of material from the surrounding fire.
Completely charred wood from retort.
Incompletely charred wood - only a small amount, and generally from the bottom of the barrel, perhaps due to a lower temperature there.
Low-tech burn demonstration by Jay, at Sherri's farm. At first, it looks a lot like a campfire.
The difference is, you keep adding wood to the outside, so the interior has little oxygen and does not burn to ash. Note there is very little smoke, even with green wood.
In the final stage (just a while after this photo), you quench the whole thing with a lot of water. A few half-burned pieces can go to the next burn, and the rest is largely char.
Preparing for burn #2, using smaller wood and some changes to the kiln.
Added a layer of firebrick at the base. Ideally, it should enclose the whole chamber, but that would take a lot of actual masonry.
More air inlets, allowing air into all four corners.
The 'chimney' is formed by the blocks themselves.
Smaller wood scraps for burn #2.
Harvesting the first of the biochar-test taro patch, results soon
Opening the kiln after burn #2
As before, the material at the bottom of the barrel (top, when inverted like this) is less charred, but everything above (below) it is completely charred
Much of the sticks that look brownish on the outside are actually completely charred black on the inside
Got the fire real hot this time, you can hear the "whoosh" of the pyrolysis gasses from the barrel joining the fire
Sifting/crushing/sorting the result. Some 1/2"-minus has direct uses. The rest will soak in nutrients to charge it, then goes through the chipper-shredder to make "charged fines" - biochar fertilizer.
Problem: the design of this kiln puts heat stress on all sides, and these regular cement blocks wear out of the stress of repeated burns, crumbling or cracking
The blocks crack and become useless at a rate of around 4 blocks per burn - clearly not sustainable
So, also trying a lower-tech solution: a simple pit, dug down and lined with blocks
firebox is 16x24x32" - that's 7.1 ft3, 53.2 gallons, ~200 liters
It's quick and simple to make a fire in the box
then when it's partly charred, tamp it down a bit
and cover with a metal sheet and a layer of soil
Returning the next day, shovel away soil and lift sheet
A few unburned logs go to the side, the rest goes into a wheelbarrow
Sifting/sorting into three sizes: 1/4"-, 1/4-1/2", and 1/2"+
Enlarging the pit, removed blocks, dug deeper and wider, re-laying more blocks
Expanded from 3x4 units to 4x6 units
Found a wider, longer sheet of metal roofing to cover the new, wider pit
Once the fire is hot, it burns with very little smoke, even as wood is added continuously
Charcoal is a fairly benign substance, but breathing a lot of any kind of dust is never a good idea
2x4x6 unit pit produced 34 gallons of char
Even larger, with an extra row of blocks, now 3 units deep
The result: roughly 68 gallons of char
Yes, it's somewhat dirty work. But not difficult or hazardous.